Engines with direct cylinder injection may utilize various configurations of pumps to deliver fuel to the cylinders. One example includes a dual stage fuel pump system. The dual stage fuel pump system may have two pumps connected in series allowing the pressure to be increased in each of two stages. The two stage system allows the size and power of the high pressure pump to be reduced, and also may allow the fuel to reach a higher injection pressure of under certain conditions. Further, such fueling systems may utilize a bypass that recirculates fuel back into the fuel tank. For example, fuel may be recirculated when the amount of fuel required by the injectors is decreased or the operation of the higher pressure pump and the injectors are stopped. Systems where the circulation occurs within or near the fuel tank are known as Mechanical Return-less Fuel Systems (MRFS).
One such example system is described by U.S. Pat. No. 6,135,090, which utilizes a pressure regulator that bypasses the low pressure pump allowing fuel to flow back into the fuel tank. In the example of '090, the regulation of the pressure at the bypass is electronically controlled by a CPU to allow the fuel system to deliver fuel to the injectors at three pressures levels.
The inventors of the present application have recognized various issues with such an approach. For example, during starting, the accuracy of pressure control at the bypass, and the response of the pressure control at the bypass, may degrade.
Further, the inventors herein have also recognized that certain engine operating conditions, such as a hot restart, may require a high lift pump pressure for a short time. One of those conditions may include operation during the engine start sequence when the fuel rail requires refilling and re-pressurization, that is, to pressurize the fuel vapor into fuel liquid. Another condition may be when the fuel lines are experiencing vapor lock (i.e. when vapor is formed in the fuel lines). The high lift pump pressure may be used to fill the fuel rail rapidly. A fully filled fuel rail may be a prerequisite for an engine-driven, positive displacement pump to be fully effective in raising fuel rail pressure.
One approach to at least partially address the above issue, as well others, may include a method for operating fuel system that supplies fuel to a plurality of injectors in an internal combustion engine via a first pump and a second pump with a bypass coupled between the first and second pump for returning fuel to a fuel tank, the pumps connected in series. The method comprises: before actuating fuel injectors during an engine start, operating the first pump, where fuel is driven through the bypass that generates increased fuel pressure delivered to the injectors increasing with said fuel flow rate driven through the bypass; and during actuation of the fuel injectors after an engine start, operating the first and second pumps, where fuel pressure is regulated at a specified pressure via bypass flow in the bypass.
For example, pressure generated by the first pump before the injectors are actuated can be used to advantage to provide increased pressure for the first injection before the second pump has generated significant pressure, or in addition to any pressure generated by the second pump. Further, by reducing the bypass fuel flow rate during injector actuation, fuel system energy efficiency can be increased while heat transfer to the fuel is reduced. In this way, it is possible to, in one example, utilize mechanical components to generate such operation, and thus increase response time during the engine start. Further, it is possible to enable a high pressure to be reached in the fuel line during engine start up, increasing the efficiency and power of the engine, and improving cold start emissions.
Thus, at one point of operation, the short-duration objective is high pressure. The above approach can exploit the fact that this requirement occurs at what would normally be maximum bypass regulator flow rate. At other points of operation, the objective is minimum or zero bypass regulator flow rate to minimize lift pump electrical power consumption.